Rhyodacitic fissure eruption in Southern Andes (Cordَn Caulle; 40.5°S) after the 1960 (Mw:9.5) Chilean earthquake: a structural interpretation

The 1960 rhyodacitic fissure eruption in the Cordón Caulle Volcanic Complex (CCVC), located in Southern Andes (40.5°S) was a unique volcanic episode. The remarkable eruption was triggered by the greatest recorded subduction-zone earthquake, starting 38 h after the main shock, 240 km inland. The structural behaviour, two compound fissures opening along a margin-oblique (NW) structure related to the Quaternary evolution of the CCVC, suggests that the prefractured nature of the upper crust in the Southern Andes was an influential condition for volcanic eruptions. From historical data and morphologic and structural analysis, we suggest that NW structures constitute pathways of steady magmatic ascent. Thus, during the great seismic event, and catalysed by the fluid pressure around a fault, it would have been reactivated allowing, initially, the propagation of a non-Andersonian dyke. Then, the silicic magma would have reached the surface by ‘seismic pumping’. Once the initial activity in the reactivated segments ceased, the local stress field would have changed, favouring the formation of new failures, this time almost parallel to the maximum horizontal stress, and promoting magma transport as Andersonian dykes. Although the rheologic characteristics of the silicic lavas erupted together with the structural behaviour and seismic features of this eruptive cycle constitute rather exceptional conditions in the Southern Andes record, the prefractured nature of the upper crust and the shifting propagation of Andersonian and non-Andersonian dykes provide a theoretical framework to analyse the neotectonics of the volcanic arc in a convergent margin.